Liquid crystal displays (LCDs) offer superior picture quality, have a smaller footprint, are more energy efficient than CRTs, and can be manufactured economically. For this reason, the market for LCDs is expanding rapidly. In terms of new products, there has been an on-going trend to introduce displays with larger active areas to compete more effectively with other display technologies. Additionally, a process that can produce larger displays can also be used to manufacture many smaller displays in parallel and thereby leverage economies of scale to reduce overall production costs.
One approach to manufacturing LCD glass sheets involves the fusion process. A key component of the fusion process is the isopipe. There is currently a demand for the production of wider LCD glass sheets. The maximum sheet glass width that can be produced by a fusion draw machine (FDM) is limited by the length of the isopipe. Thus, longer isopipes are needed to produce wider glass sheets.
Certain isopipes currently used to produce LCD glass sheets are made from a zircon refractory. High temperature creep of the isopipe is believed to limit its operational lifetime. Display glass drawn on an isopipe with excessive creep cannot produce a sheet of uniform thickness. Creep deformation leads to an uneven distribution of glass flow. Hence, a reduction in the intrinsic creep rate of the refractory material used to manufacture the isopipe can result in the use of a wider isopipe, extend the fusion draw process to higher temperature glasses, and extend service life of the isopipe in the current process to minimize process down time and replacement costs.
In the case of an isopipe, the creep rate is a function of the material itself as well as the length and height of the isopipe. Longer isopipes are more prone to failure due to creep. Failure of the isopipe can interrupt the production of glass and lead to prolonged, undesired downtime of the manufacture process.
The creep rate of a polycrystalline ceramic body such as an isopipe is influenced by factors that can be categorized as either microstructural (e.g., porosity, grain size, grain shape, and arrangement of second phases) or chemical such as stoichiometry and concentration of impurities. The microstructure and chemical composition of the ceramic body are themselves dependent upon processing history, which includes initial particle size of the powders, sintering temperature, and contaminants. These same factors play a role in the ability of the material to meet other requirements for an isopipe such as strength. Thus, it would be desirable to modify the microstructural properties and chemical composition of a refractory material such that the creep rate is reduced and other properties remain satisfactory. Ultimately, the refractory material would be used to produce isopipes and other components used in the glass-making process. The methods and compositions described herein address this need.